Project Summary/Abstract: The incidence of infections due to highly resistant Acinetobacter baumannii and other GNB is increasing. True pan-drug resistant (PDR) strains have been reported. Unfortunately, the newly approved antimicrobials ceftolozane-tazobactam and ceftazidime-avibactam are poorly active against resistant A. baumannii. The need to identify new antimicrobials active against A. baumannii and other GNB is pressing. We have demonstrated that A. baumannii dihydroorotate dehydrogenase (AbDHODH) is in vivo essential in rat and mouse infection models. Importantly, DHODH is a validated drug target in Plasmodium falciparum with an antimalarial specific compound evaluated in a recently completed Phase 2 clinical trial (NCT02123290). While DHODH has been verified as a druggable target in Plasmodium, it has received scant attention as an antibacterial target and has not been considered for A. baumannii. The development of antimicrobials that target bacterial DHODH would represent a new class predicted to be active against resistant A. baumannii and perhaps other GNB. We have screened purified AbDHODH against a unique library of 450 chemically elaborated drug-like derivatives developed for testing against P. falciparum DHODH (PfDHODH). This resulted in the identification of 18 triazolopyrimidine and imidazopyrimidine drug-like derivatives with nM activity (11 nM-940 nM). We have obtained a co-crystal structure with the most potent compound. All three compounds assessed to date against live A. baumannii are active. The most active compound (11 nM activity) had a minimal inhibitory concentration (MIC) of ? 1 g/ml against 11/12 geographically diverse A. baumannii strains tested (8 of which were resistant to meropenem), with an MIC of 2 g/ml for the remaining strain. Most importantly a second compound, that was less active (110 nM activity) but available in sufficient quantity, conferred significant protection in the neutropenic mouse thigh infection model. Our experience developing antimalarial therapeutics targeting PfDHODH to the stage of clinical development will facilitate the completion these aims. In aim 1, hit expansion through the synthesis of analogs of the identified scaffolds and subsequent assessment will enable further insight into liabilities. These data will enable optimization to best suit the new application of treating A. baumannii infections. In Aim 2, hit to lead progression and lead optimization of AbDHODH inhibitors will proceed via a combination of iterative medicine chemistry, SAR analysis, crystallography, toxicity studies, in vitro ADME analysis and proof-of-concept in vivo efficacy studies with the objective to identify a late lead compound with the properties needed to advance to preclinical development. Aim 3 will assess the potential for lead compounds to develop resistance. Aim 4 with expand upon biologic, pharmacologic and toxicity analyses to enable the nomination of a pre-clinical candidate for the treatment of A. baumanni and potentially other high-risk bacterial infections.